Modern blast furnaces are designed to operate at a nominal furnace pressure in the range of 1-3 bar above atmospheric. To protect the furnace and its auxiliary equipment from any sudden gas pressure surge, bleeder valves are typically provided at the top of a blast furnace. Such pressure surges occur e.g. due to slips or hangings in the burden or due to problems in blower operation or pressure regulation. Accordingly, bleeder valves act as safety relieve valves that open at a given threshold to relieve excess pressure.
In a typical blast furnace installation, two bleeder valves are arranged at the furnace top, each at the upper end of a respective bleeder pipe that rises upwards from and connects to the uppermost point of a pair of uptakes. The uptakes are ducts from which blast furnace gas is withdrawn for further use from the furnace top cone. The uptakes normally join above the furnace charging installation, each being connected to a duct called downcomer, through which the blast furnace gas descends to a treatment installation. An additional bleeder valve may be connected to the secondary blast furnace gas cleaning installation, e.g. to a scrubber.
Depending on the design, a bleeder valve can be operated both automatically and manually. Some bleeder valves are operated hydraulically or pneumatically through pressure lines that cause the valve to open in case of excess pressure. Normally, the latter type also allows manual opening of the valve, e.g. in case of a furnace shutdown. Another type of bleeder valves are usually counterweighted or spring-biased so that they will open without the need of external power when the pressure significantly exceeds normal operating pressure. Improved bleeder valve designs developed by PAUL WURTH, commonly referred to as “coffee-pot” bleeders, combine both functionalities, i.e. controlled full opening by use of external power and independent emergency opening by action of the furnace pressure against resilient bias. Examples of this type of bleeder valve are disclosed in U.S. Pat. Nos. 3,601,357 and 4,158,367. A “coffee-pot” bleeder having an improved design of the obturator is disclosed in international patent application no. WO 2007/090747. Irrespective of their design, the bleeder valves are usually configured to release blast furnace off-gas to the atmosphere.
In a conventional process, blast furnace gas typically contains around 45-55% of N2, around 15-25% of CO, around 15-25% of CO2 and around 1-10% of H2. Under certain circumstances and depending on the process, the volume fraction of carbon-monoxide may become greater than 25% whereas the volume fraction of hydrogen may become greater than 10%, reaching 15% of H2 for instance in a blast furnace with natural gas injection. Since CO is highly toxic and since CO and H2 are both highly flammable, releasing blast-furnace off-gas to the atmosphere is hazardous. In fact, off-gas released through a bleeder valve sometimes ignites, e.g. due to incandescent pieces of burden expelled through the bleeder valve, resulting in a meters-long darting flame. In rare cases, explosive clouds have built up above the furnace and led to deflagration explosions. Apart from posing threats relating to uncontrolled combustion and poisoning, off-gas released through bleeder valves obviously causes pollution.
Whereas the bleeder valves are shut at normal operating conditions of the furnace, each time the bleeder valves do open, a considerable amount of hazardous blast furnace off-gas is released to the atmosphere. Moreover, in case of abnormal furnace operation, the bleeder valves may open several times a day. Even in case of a stable process, gas may be released several times a year. Accordingly, there is a need for eliminating or at least reducing the risks caused by blast furnace off-gas emission from bleeder valves.
FIG. 1 shows an arrangement according to Japanese patent application no. 54 107 806, which aims at making blast furnace off-gas discharged from bleeder pipes harmless by burning the off-gas. As seen in FIG. 1, the outlet of several bleeder valves 2 is connected to a common collecting pipe 3 that leads off-gas to a combustion chamber 4. Carbon containing gas is fed by line 5 to the combustion chamber 4 for completely burning the blast furnace off-gas in the chamber 4 before it is released to the atmosphere. This arrangement has a safety installation including fire-extinguishing means for avoiding a backflash of fire from the combustion chamber 4 into the furnace. To this effect, a pressure sensor 7 is provided in the collecting pipe 3. When the sensed pressure in the collecting pipe 3 becomes “negative”, a controller 8 shuts off a valve 10 in the carbon gas feed line and opens a valve 11 to inject fire-fighting steam into the collecting pipe 3. When “positive” pressure is sensed, i.e. when off-gas is released through the bleeder valves 2, the steam valve 11 is closed and the carbon gas valve 10 is opened so that combustion of the off-gas in the combustion chamber 4 can be ignited by an ignition device 6 arranged in the combustion chamber 4.
As will be understood, an arrangement according to JP 54 107 806 is rather complex and has considerable installation costs, especially if it is to be adapted to modern high-pressure blast furnace plants. In fact, a collecting pipe designed for collecting bleeder off-gases, as proposed in JP 54 107 806, has to have both a configuration and a support structure that are very robust in order to withstand the considerable forces caused by the bleeder outflow, which may reach supersonic speeds in modern furnaces, the high temperatures and abrasion due to the dust content. It is thus even questionable whether such an arrangement is practically feasible in a modern blast furnace plant.
Another device for burning off-gases in a blast furnace is disclosed in U.S. Pat. No. 3,907,261. Contrary to the afore-mentioned prior art, this device does not relate to a bleeder valve but to a blow-off discharge valve mounted on the downcomer of a blast furnace. Such valves, often called “shut-down discharge hats”, are used only during furnace shutdown to release the comparatively lower quantity of top gas produced during a shutdown. The valve of U.S. Pat. No. 3,907,26 comprises a valve flap that opens inwardly into the valve housing and a tubular member that is axially inserted into the valve housing when the valve flap is opened. The tubular member protects the valve flap and the valve seat from direct exposure to blast furnace gas. The tubular member can be equipped with a flaring device that comprises a tubular extension on the tubular member and a coaxial outer jacket. The extension and jacket define a combustion chamber that is open at its lower end to permit ingress of sufficient air to support combustion. The flaring device further includes an igniter fixed to the outer jacket and extending into the combustion chamber and an injection nozzle coaxially disposed inside the tubular extension for injecting liquid or gaseous fuel. As will be understood, a shutdown discharge hat with a flaring device according to U.S. Pat. No. 3,907,26 is not suitable for use as a bleeder valve for explosion prevention. Among others, its inwardly opening configuration precludes a reliable relief of sudden pressure excesses during furnace operation.